Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. High-energy ion implanters are used for deep implants into a substrate. Such deep implants are required to create, for example, retrograde wells. Eaton GSD/HE and GSD/VHE ion implanters are examples of such high-energy implanters. These implanters can provide ion beams at energy levels up to 5 MeV (million electron volts). U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, Eaton Corporation, and describing such an high-energy ion implanter, is incorporated by reference herein as if fully set forth.
A block diagram of a typical high-energy ion implanter 10 is shown in FIG. 1. The implanter 10 comprises three sections or subsystems: a terminal 12 including an ion source 14 powered by a high-voltage supply 16 to produce an ion beam 17 of desired current and energy; an end station 18 which contains a rotating disc 20 carrying wafers W to be implanted by the ion beam; and a beamline assembly 22, located between the terminal 12 and the end station 18, which contains a mass analysis magnet 24 and a radio frequency (RF) linear accelerator (linac) 26. A final energy magnet (not shown in FIG. 1) may be positioned between the linac 26 and the rotating disc.
The RF linac 26 comprises a series of resonator modules 30a through 30n, each of which functions to further accelerate ions beyond the energies they achieve from a previous module. FIG. 2 shows a known type of resonator module 30, comprising a large inductive coil L having a circular cross section and being contained within a resonator cavity housing 31 (i.e., a "tank" circuit). A radio frequency (RF) signal is capacitively coupled to a high-voltage end of the inductor L via capacitor C.sub.c. An accelerating electrode 32 is directly coupled to the high-voltage end of the inductor L. Each accelerating electrode 32 is mounted between two grounded electrodes 34 and 36, and separated by gaps 38 and 40, respectively.
FIG. 3 shows a simple lumped parameter equivalent circuit for the resonator geometry of FIG. 2. The capacitance C includes the capacitance of the high voltage electrode with respect to ground, the stray capacitance of the coil and electrode stem with respect to ground, and the inter-turn coil capacitance.
Values for C and L are chosen for the circuit to achieve a state of resonance so that a sinusoidal voltage of large amplitude may be achieved at the accelerating electrode 32. The accelerating electrode 32 and the ground electrodes 34 and 36 operate in a known "push-pull" manner to accelerate the ion beam passing therethrough, which has been "bunched" into "packets". During the negative half cycle of the RF sinusoidal electrode voltage, a positively charged ion packet is accelerated (pulled by the accelerating electrode 32) from the first grounded electrode 34 across gap 38. At the transition point in the sinusoidal cycle, wherein the electrode 32 is neutral, the packet drifts through the electrode 32 (also referred to as a "drift tube") at constant velocity.
During the positive half cycle of the RF sinusoidal electrode voltage, positively charged ion packets are further accelerated (pushed by the accelerating electrode 32) toward the second grounded electrode 36 across gap 40. This push-pull acceleration mechanism is repeated at subsequent resonator modules having accelerating electrodes that also oscillate at a high-voltage radio frequency, thereby further accelerating the ion beam packets by adding energy thereto. The RF phase of successive accelerating electrodes in the modules is independently adjusted to insure that each packet of ions arrives at the appropriate gap at a time in the RF cycle that will achieve maximum acceleration.
Referring to FIG. 3, it is convenient for analysis to replace the three circuit values R, L and C by the parameters .omega. (the resonant frequency), Q (the quality factor), and Z (the characteristic impedance), where: .omega.=(LC).sup.-1/2, Q=R/(.omega.L), and Z=.omega.L=1/(.omega.C)=(LC).sup.1/2. Note that .omega. is the radial frequency, equal to 2 .pi. times the conventional frequency (Hertz).
To minimize the power required to obtain a given electrode voltage, the product of the quality factor Q and the characteristic impedance Z must be maximized. Prior art resonators such as that shown in FIG. 4 are designed using known design principles for high Q resonators. Such designs utilize a circular cross section conductor for the coil. By utilizing a rectangular cross section conductor, as is contemplated by the present invention, with the short dimension parallel to the coil axis 47, higher impedance coils may be realized while still maintaining a high quality factor Q. The shorter conductor dimension parallel to the coil axis allows smaller winding pitch, i.e., a shorter coil, which has less capacitance with respect to ground (the resonator housing 31). Thus, the ratio of the coil inductance to the coil capacitance is increased.